Traveling-wave linear cavity laser

ABSTRACT

A linear cavity design to produce a traveling wave operation as in a ring laser without the ring cavity design. All fiber configurations may be used to implement fiber lasers based on the linear cavity design.

This application is a continuation-in-part application of and claims thebenefit of a U.S. patent application Ser. No. 10/666,139 entitled“Traveling-Wave Lasers with a Linear Cavity” and filed Sep. 18, 2003 nowU.S. Pat. No. 7,006,550, which further claims the priority of U.S.Provisional Application No. 60/411,856 filed on Sep. 18, 2002.

The entire disclosures of U.S. Patent publication No. US 2004-0057471 A1and U.S. Pat. No. 7,006,550 for the U.S. patent application Ser. No.10/666,139 and the U.S. Provisional Application No. 60/411,856 areincorporated by reference as part of this application.

BACKGROUND

This application relates to lasers, and in particular, to single-modelasers and fiber lasers.

Various applications may require the laser wavelength of a laser to bestabilized at a specific wavelength. In opticalwavelength-division-multiplexed (WDM) systems, for example, the signalwavelengths of different optical WDM channels need to be maintained atdesignated WDM standard wavelengths according to a wavelength standardsuch as the WDM wavelengths established by the InternationalTelecommunication Union (ITU). Other applications such as spectroscopicmeasurements may also use stabilized lasers to excite selected atomic ormolecular transitions.

SUMMARY

This application includes an exemplary linear laser cavity design toprovide a traveling wave operation that is usually achieved in a ringcavity, without having an actual ring cavity. Implementations ofsingle-wavelength fiber lasers based on the cavity design, andtechniques for tuning the laser wavelength of such lasers are alsodescribed. In one implementation, such a linear laser cavity may includea linear optical cavity having first and second polarization sensitiveoptical reflectors to reflect light at a laser wavelength andpolarization state and to transmit light at a pump wavelength differentfrom the laser wavelength. A laser gain section is provided in thelinear optical cavity to produce optical gain at the laser wavelength byabsorbing the pump light. Notably, first and second optical polarizationrotating elements are in said linear optical cavity and are respectivelylocated on two opposite sides of the laser gain section to makecounter-propagating light beams at the laser wavelength to haveorthogonal polarizations. In some implementations, an optical filter mayalso be placed in the linear optical cavity to select a laser mode forlight at the laser wavelength to transmit and being transparent to lightat the pump wavelength.

Based on the above design, all-fiber devices may be constructed to havea linear fiber cavity. For example, a fiber device may include a firstpolarization-maintaining (PM) fiber section having a first fiber gratingto reflect light at the laser wavelength with its polarization alignedalong one principle axis of the PM fiber section and to transmit lightat the pump wavelength different from the laser wavelength. A dopedfiber gain section, e.g., using a single-mode fiber, is used to produceoptical gain at the laser wavelength by absorbing the pump light. Aquarter-wave plate is optically coupled between the first PM fibersection and a first side of the fiber gain section. A second PM fibersection is provided to have a second fiber grating to reflect light atthe laser wavelength with polarization orthogonal to the first fibergrating and to transmit light at the pump wavelength. Athree-quarter-wave plate is optically coupled between the second PMfiber section and a second side of the fiber gain section. In addition,an optical filter is optically coupled between the first and secondfiber gratings to produce a peak transmission for light at the laserwavelength and being transparent to light at the pump wavelength. Thedoped fiber gain section may be made of a doped silica fiber or otherdoped fibers.

Fiber lasers based on this linear laser cavity design may be stabilizedat a specified laser wavelength with a high side mode suppression ratio,narrow linewidth, shot noise limited AM noise and high polarizationextinction ratio. Such fiber lasers can also be very efficient due tothe elimination of spatial hole burning, may have a compact athermalpackage and may be manufactured at a low cost. In addition, such fiberlasers may be tunable in laser wavelength.

In one implementation, a device has a linear optical cavity and a lasergain section in the cavity. The linear optical cavity has first andsecond optical reflectors to reflect light at a laser wavelength and totransmit light at a pump wavelength different from the laser wavelength.The first reflector selectively reflects only light linearly polarizedin a first direction and said second reflector selectively reflectslight linearly polarized in a second direction. The laser gain sectionis used to produce optical gain at the laser wavelength by absorbing thepump light. This device also includes first and second opticalpolarization elements in the linear optical cavity and respectivelylocated on two opposite sides of the laser gain section to makecounter-propagating light beams at the laser wavelength reflected fromsaid first and second optical reflectors to have orthogonalpolarizations between the first and second optical polarizationelements. In addition, an optical filter is located in the linearoptical cavity between the first and second optical polarizationelements to selectively transmit a single mode at the laser wavelengthsupported by the linear optical cavity.

In another implementation, a device includes a firstpolarization-maintaining (PM) fiber having a first fiber grating, adoped fiber gain section, a quarter-wave plate, a second PM fibersection having a second fiber grating, a three-quarter-wave plate, andan optical filter in the cavity formed by the first and second fibergratings. The first PM fiber section has a first fiber grating toreflect light at a laser wavelength and to transmit light at a pumpwavelength different from the laser wavelength the doped fiber gainsection is used to produce optical gain at the laser wavelength byabsorbing light at the pump wavelength from the first PM fiber section.The quarter-wave plate is optically coupled between the first PM fibersection and a first side of the doped fiber gain section and oriented tocovert light from the first fiber grating into a first circularlypolarized light. The second PM fiber section has a second fiber gratingto reflect light at the laser wavelength and to transmit light at thepump wavelength. The three-quarter-wave plate is optically coupledbetween the second PM fiber section and a second side of the doped fibergain section, and oriented to convert light from the second fibergrating into a second circularly polarized light orthogonal to the firstcircularly polarized light. The optical filter optically is used toproduce a peak transmission for light at the laser wavelength and beingtransparent to light at the pump wavelength.

In yet another implementation, a fiber line is provided to have an inputend which receives a pump beam at a pump wavelength and an output endwhich exports a residual of the pump beam and a laser beam at a laserwavelength shorter than the pump wavelength. The fiber line comprising afirst fiber laser and a second fiber laser in series to share the samepump light in the fiber line. The first laser includes first and secondfiber gratings spaced away from each other to form a linear fiberoptical cavity and each configured to reflect light at the laserwavelength along a linear polarization direction and to transmit lightat the pump wavelength. The first laser also includes a doped fiber gainsection between the first and said second fiber gratings to absorb thepump beam and to produce and amplify the laser beam. A first fiberpolarization element is coupled between said first fiber grating andsaid doped fiber gain section and configured to convert light reflectedfrom said first fiber grating at the laser wavelength into a firstcircularly polarized light. A second fiber polarization element iscoupled between said second fiber grating and said doped fiber gainsection and to convert light at the laser wavelength reflected from saidsecond fiber grating into a second circularly polarized light orthogonalto polarization of said fist circularly polarized light. In addition, anoptical fiber bandpass filter is optically coupled between said firstand said second fiber gratings. This optical fiber bandpass filter istransparent to the pump beam and selects a laser mode at the laserwavelength to transmit while rejecting other laser modes at the laserwavelength.

A method according to one implementation includes the following steps. Alinear optical cavity is formed in a fiber strand with first and secondfiber Bragg reflectors respectively formed n first and secondpolarization-maintaining fibers spaced from each other and a fiber gainsection between the fiber Bragg reflectors. Each fiber Bragg reflectorreflects light at a laser wavelength and to transmit light at a pumpwavelength different from the laser wavelength. The fiber gain sectionabsorbs light at the pump wavelength to produce an optical gain at thelaser wavelength. An intra-cavity filter is provided in the linearoptical cavity to select a single cavity mode to lase. The lightpolarization in the linear optical cavity is then controlled to makecounter-propagating light beams at the laser wavelength to haveorthogonal polarizations in at least the fiber gain section and to makesaid intra-cavity filter to transmit light reflected from said first andsaid second fiber Bragg reflectors.

These and other implementations and associated methods are described ingreater detail with reference to the drawings, the detailed description,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one example of a linear laser cavity design to support atraveling wave operation as in a ring laser cavity, where the counterpropagating beams 102 and 103 are either both right-hand circularlypolarized (RCP) as labeled or light-hand circularly polarized (LCP).

FIG. 2 shows one exemplary tunable laser based on the linear cavitydesign in FIG. 1.

FIG. 3 shows an athermal packaging design for a fiber laser based on thelinear cavity design in FIG. 1.

FIGS. 4 and 5 are measurements of a fiber laser based on the linearlaser cavity design in FIG. 1.

FIG. 6A shows an exemplary laser system that connects lasers in FIG. 1in series to share the same optical pump.

FIG. 6B shows an example of a fiber laser based on the design in FIG. 1to include a downstream fiber amplifier that is optically pumped by thetransmitted pump power from the fiber laser.

FIG. 7 shows another example of a traveling-wave fiber laser with alinear laser cavity.

FIGS. 8, 9, and 10 show spectra of exemplary elements in the laser inFIG. 7.

FIGS. 11A and 11B show orientations of a quarter wave plate and a threequarter wave plate that may be used in FIG. 1.

FIGS. 12A and 12B show orientations of two different quarter wave platesthat may be used in FIG. 1.

FIGS. 13A and 13B show two laser designs without an intracavity opticalfilter.

FIG. 14 shows an example of a traveling-wave fiber laser with a linearlaser cavity using a non-polarization maintaining fiber section having anon-polarization maintaining fiber grating reflector as one side of thelaser cavity.

FIG. 15 shows an example of a traveling-wave fiber laser with a linearlaser cavity using a birefringent laser gain medium inside the lasercavity and two non-polarization maintaining fiber sections each having anon-polarization maintaining fiber grating reflector to form the lasercavity.

FIG. 16 shows an example of a traveling-wave fiber laser with a linearlaser cavity using a birefringent laser gain medium between twowaveplates inside the linear laser cavity where one of the twowaveplates is designed to offset the optical birefringence in the lasergain medium.

FIG. 17 shows an example of a traveling-wave fiber laser with a linearlaser cavity using a polarization-preserving twisted single-mode fibergain loop inside the laser cavity.

DETAILED DESCRIPTION

FIG. 1 shows an exemplary laser 100 with a linear laser cavity designedto lase at a desired laser wavelength (λ_(L)). Two reflectors 111 and121, such as fiber Bragg gratings, are positioned in an optical path andspaced from each other to form a Fabry-Perot laser cavity as the linearlaser cavity where optical energy at the laser wavelength (λ_(L)) isreflected between them. The reflectors 111 and 121 are opticallyreflective at a band around the laser wavelength (λ_(L)) and areoptically transmissive at other wavelengths. For example, either one orboth reflectors 111 and 121 may be transmissive to light at a pumpwavelength (λ_(p)) for optically pumping a laser gain medium in theoptical path between the reflectors 111 and 121. One of the reflectors111 and 121 may be partially transmissive to light at the band aroundthe laser wavelength (λ_(L)), e.g., the reflector 121, to produce alaser output 105. The reflector 111 in this example may be completelyreflective at the laser wavelength (λ_(L)). Alternatively, thepump-receiving reflector 111 may be partially transmissive and thereflector 121 may be completely reflective at the laser wavelength(λ_(L)) to produce the desired laser output.

In some implementations where transmitted pump beam is not needed, anadditional reflector reflective to the pump wavelength may be formed inthe laser 100 to reflect the pump light transmitted through the gainsection back to the gain section to increase the pump efficiency and orto stabilize the pump wavelength. A pump-reflecting fiber Bragg gratingmay be used to achieve this. When the reflector 121 is a fiber Bragggrating in a strand of fiber, the pump reflecting fiber Bragg gratingmay be formed in the same fiber that is either spatially overlapped withthe grating reflector 121 or separated from the grating reflector 121.

The laser 100 is designed in part to make the counter-propagating laserbeams have orthogonal polarizations. In this regard, within the lasercavity, a polarization control mechanism is implemented to insure theproper orthogonal polarizations in counter-propagating beams. In theexamples described here, two polarization elements are positioned at twoopposite ends as this polarization control mechanism. In the example inFIG. 1, a quarter-wave plate 140 for light at the laser wavelength isplaced at one end inside the laser cavity. A three-quarter-wave plate170 for light at the laser wavelength is placed at the other end insidethe laser cavity. A laser gain section 150 is placed in the optical pathbetween the wave plates 140 and 170 to absorb a pump beam 101 and toproduce an optical gain at the laser wavelength (λ_(L)). The positionsof the waveplates 140 and 170 may be exchanged. As will be seen below,the waveplates 140 and 170 are placed within the laser cavity to atleast eliminate formation of standing waves associated with theundesired spatial hole burning and to mitigate the composite cavityeffects associated with insertion of an intra-cavity optical filter.

In order to mitigate the composite cavity effects, the first and secondoptical reflectors 111 and 121 may be sensitive to polarization in theiroperations and reflect laser light at the laser wavelength and operatein combination with the wave plates 140 and 170, respectively. In oneimplementation when the reflectors 111 and 121 are formed of the samematerial, reflectors 111 and 121 may be designed to reflect laser lightwith orthogonal polarizations. Hence, the first optical reflector 111may reflect the laser light at a first polarization while the secondoptical reflector 121 may reflect the laser light of the same opticalfrequency at a second, orthogonal polarization.

The linear cavity formed by the reflectors 111 and 121 generally allowsfor multiple modes to lase. The laser 100 may operate in a single cavitymode. In this regards, an intra-cavity optical filter 160, such as asingle mode Fabry-Perot filter or a comb filter, is placed between thelaser gain medium 150 and the three-quarter-wave plate 170 to produce anarrow transmission at the laser wavelength to select only one of thelaser modes to lase. In general, the filter 160 may be located inanywhere inside the cavity, e.g., in the gain section 150. The filter160 may include two fiber Bragg grating reflectors 161 and 162 toreflect light at the laser wavelength at a band slightly larger than thebandwidth of the reflection bandwidth of 111 and 121 and transmit lightat the laser mode frequency. In some implementations, the filter 160 mayinclude more than two Bragg gratings to flatten the top of thetransmission spectral response at the laser mode frequency. A flat topoptical filter may be used as the filter 160 to mitigate any mismatchbetween the laser and the peak transmission of the filter 160.

The presence of this intra-cavity filter 160, however, may potentiallycreate composite optical cavities in the laser 100. For example, thereflector 111 and the reflector 161 may effectuate one resonator forlight that is reflected by the reflectors 111 and 160 (e.g., the side161). Similarly, reflectors 160 (e.g., the side 162) and 121 mayeffectuate another optical resonator. Such multiple composite resonatorsare known to adversely affect the optical operations of the laser 100.In the illustrated example in FIG. 1, the cavity design uses the twowaveplates 140 and 170 and the polarization-sensitive reflectors 111 and121 to eliminate undesired effects of composite cavities that wouldotherwise be present due to composite cavities formed by the reflectors111, 121 and grating reflectors in the filter 160 gratings. Such effectsare undesirable because the matching of modes due to the compositecavities can be technically difficult. In addition, the intra-cavitylight at the laser wavelength is circulating between the reflectors 111and 121 to be amplified by the gain medium 150 without interfering witheach other to form a standing wave in the laser gain medium 150. The twowaveplates 140 and 170, when situated as shown in FIG. 1 or in analternative configuration where their positions are exchanged, operateto make the polarization states of counter-propagating beams at thelaser wavelength to be orthogonal between the reflectors 111 and 121.Hence, the interference between counter-propagating beams and theassociated adverse spatial hole-burning problem may be eliminated.Therefore, this linear laser cavity allows for traveling wave operationsimilar to a ring laser without having a conventional ring cavitydesign.

The polarization-sensitive reflectors 111 and 121 may be designed toreflect linearly-polarized light in specified polarization directions,e.g., by using fiber grating reflectors in PM fibers. Under this design,the laser 100 in FIG. 1 uses the two wave plates 140 and 170 totransform the linearly-polarized light reflected from thepolarization-sensitive reflectors 111 121 into circularly polarizedlight in the portion of the cavity between the wave plates 140 and 170in a way to essentially eliminate both the spatial hole burning problemdue to interference of counter-propagating laser beams and the compositecavities due to the presence of the intra-cavity filter 160.

As an example, assume the polarization sensitive reflector 111 reflectsonly the light at the laser wavelength that is linearly polarized alongthe x direction and the polarization sensitive reflector 121 reflectsonly the light at the laser wavelength that is linearly polarized alongthe orthogonal y direction. In addition, it is assumed that the waveplate 140 transforms the x-polarized light reflected from the reflector111 into a right-hand circularly polarized (RCP) light 102 and the waveplate 170 transforms the y-polarized light reflected from the reflector111 into a right-hand circularly polarized light 103. Because the beams102 and 103 are in opposite directions, they are orthogonal to eachother in polarization. Upon transmitting through the wave plate 140,this light becomes linearly polarized along the y direction. Because thereflector 111 only reflect light in the x polarization, the y-polarizedlight reflected from the filter 160 will transmit through the reflector111 without being reflected. Hence, the reflector 111 and the filter 160cannot form an optical resonator. Similarly, the reflector 121 and thefilter 160 cannot form a resonator either. In addition, the counterpropagating beams between the wave plates 140 and 170 are RCP beams thatdo not interfere to form a standing wave. Thus, the laser 100 does nothave the spatial hole burning as in some other lasers with linearcavities.

The cavity design of the laser 100 may be implemented in various ways.As illustrated, one particular implementation of the laser 100 is anall-fiber design as a single-mode fiber laser. In the fiberimplementation, fibers are used to form various components in theoptical path of the laser 100. The reflectors 111, 161, 162, and 121,for example, may be fiber Bragg gratings formed within the respectivefibers. The laser gain medium 150 may be a doped fiber section that hasdesired optical transitions within the gain spectral range for laseroscillation. Atomic transitions in rare-earth ions (e.g., Er, Yb, etc.)or other active ions, for example, may be used to produce lasers fromvisible wavelengths to far infrared wavelengths. Er-doped fiberamplifiers (EDFA) for producing optical signals at 1.55 microns areuseful in optical fiber communication applications because the opticalloss in the commonly used silica fibers is minimum at about 1.55microns. In another implementation, the gain section 150 can also be adoped, non-silica fiber including doped fibers using phosphate, fluorideor bismuth as the host materials. Furthermore, the gain section 150 mayalso be a semiconductor optical amplifier. The waveplates 140 and 170may also be formed by two polarization-maintaining fibers with theirprincipal polarization axes properly oriented and with proper lengths.Different fibers used in the laser 100 may be spliced together so thatthe entire laser 100 is essentially one piece of fiber. Such a fiberlaser can be designed to be light, compact, inexpensive to fabricate,and may provide highly stable single-mode single polarization laseroperation with a narrow linewidth, high side mode suppression and signalto noise ratio. In addition, the laser 100, whether or not in the fiberimplementation, may include a cavity control unit to tune the outputlaser wavelength.

The fiber laser 100 may be optically pumped by a pump beam 101 that iscoupled into the laser cavity from one side, e.g., through thefiber-grating reflector 111 as illustrated. Alternatively, a sidepumping configuration may be used to optically pump the gain section 150from the side. A suitable pump wavelength (λ_(p)) is preferably at awavelength outside the gain spectral range of the doped region 150 andis usually shorter than the laser wavelength (λ_(L)). For example, Er⁺³,other rare earth ions, and other suitable ions can be doped in varioushost fiber materials such as, silica, fluoride, phosphate, and bismuthfibers to produce laser oscillations, e.g., at 1.55 microns whenoptically pumped at 980 nm or 1480 nm. A pump light source that producesthe pump beam 101 may include a LED or a laser diode to produce pumplight at one or more pump wavelengths in resonance with at least oneoptical transition in the doped fiber gain medium 150 for producingphotons at the laser wavelength. Since fiber grating reflectors 111,161, 162, and 121 may be reflective only at the laser wavelength thatsatisfies the Bragg phase matching condition and are transparent tolight at other wavelengths, the remaining of the pump beam 101 at λ_(p)that is not absorbed by the gain medium 150 transmits through the laser100 as a transmitted pump beam 101A. This transmitted pump beam 101A maybe used to amplify the laser output with the addition of a fiber gainsection at the output by optically pumping the additional fiber gainsection. The transmitted pump beam 101A may also be used to pump anotherfiber laser based on the same design so that the single pump beam 101may be shared by two or more such fiber lasers optically connected inseries. In certain implementations, the pump beam 101A may be reflectedback into the fiber laser to increase its efficiency and or to stabilizethe pump wavelength at an optimum value.

The fiber laser 100 may be generally divided into three parts based onthe types of fibers used: an input polarization-maintaining (PM) fiberpart 110 with the pump-receiving reflector 111, a single-mode fiber part130 with the doped fiber gain section 150 and the fiber Fabry-Perotfilter 160, and another PM fiber part 120 with the reflector 121. Asillustrated, the waveplates 140 and 170 may be connected between thefiber sections 110 and 130, and between fiber sections 130 and 120,respectively, at the respective fiber connecting points 112 and 122.Alternatively, the waveplate 140 may be formed within the PM fiber 110and the waveplate 170 may be formed within the PM fiber 120. The fibergrating reflector 111 may be formed in the PM fiber 110 by, e.g.,imprinting with UV exposure. The fiber grating reflector 111 may bepartially transmitting having a reflectivity of about or close to 100%at the laser wavelength. A short strand of PM fiber, e.g., about 1 mmwith proper indices of refraction in the two orthogonal polarizations,may be spliced to the PM fiber 110 with the principal axis at 45 degreeswith respect to the PM fiber axis in the fiber 110 to act as thequarter-wave plate 140 at the laser wavelength (e.g., at 1550 nm).

The gain medium 150 may include a few cm of single mode fiber highlydoped with Er atoms or other rare earth species. This fiber segment isspliced to the PM fiber for the waveplate 140. Another section of singlemode fiber with in-fiber resonator grating in a strand of Silica fiberis used to form a high finesse filter 160 with one side spliced to thefiber gain medium 150.

In one implementation of the three-quarter-wave plate 170, a section ofPM fiber (e.g., about 3 mm) may be spliced at 45 degrees to the PM fiber120 as the three-quarter-wave plate 170. In general, the two principalpolarization axes of each of the waveplates 140 and 170 are aligned at45 degrees with the two principal polarization axes of the adjacent PMfiber (110 or 120). The fiber grating 121 in the PM fiber 120 may have areflectivity close to 80%, for example, to produce the laser output 105.

The wave plates 140 and 170 may be used to prevent the formation ofoptical cavities between gratings 111 and 121 with the gratings 161, 162in the filter 160, respectively. As described above, the wave plates 140and 170 can also eliminate the formation of a standing wave in the gainmedium 150 and thus the spatial hole burning therein which wouldotherwise be present in such a linear cavity. Therefore, the opticalgain in the gain medium 150 can be efficiently used to achieve highlaser output power and a high signal to noise ratio.

Notably, the PM fiber gratings 111 and 121 are formed in PM fibers withoptical birefringence and thus can create double reflection peaks due tothe slightly different indices of refraction along the principal axes ofeach PM fiber. In one implementation such as when the PM fiber gratings111 and 121 are made of the same PM fiber material, the high frequencypeak of the fiber grating 111 can be aligned in frequency both with thelow frequency peak of the fiber grating 121 and with the singletransmission peak of the Fabry-Perot filter 160. Under this design, thefiber grating reflectors 111 and 121 are polarization-sensitivereflectors where the reflector 111 reflects laser light at a selectedlaser wavelength with a polarization along a first polarization and thereflector 121 reflects laser light at the same selected laser wavelengthwith a second polarization perpendicular to the first polarization. Whenpumped by the pump light 101 at a desired pump wavelength, e.g., a 980nm or 1480 nm, the fiber laser 100 can sustain a single mode laseroperation at a frequency defined by the transmission peak of the filter160, e.g., around 1550 nm. The laser output 105 is linearly polarizedalong the principal axis at the output PM fiber 120.

FIG. 4 shows a typical transmission peak of a laser mode in an exemplaryfiber Fabry-Perot (FP) filter 160. FIG. 5 shows the relative alignmentof different peaks of exemplary fiber grating reflectors 111, 121, andthe filter 160 in the laser 100 that are represented by red, green, andblue traces, respectively. The transmission peak of the filter 160 ismarked by a numeral 500. The reflection spectrum for the gratingreflector 111 has peaks 521 and 522 for light of the same frequency withfirst and second orthogonal polarizations, respectively. Similarly, thereflection spectrum for the grating reflector 121 has peaks 511 and 512for light of the same frequency with first and second orthogonalpolarizations, respectively. The peak 512 for light in the secondpolarization of the grating reflector 121 overlaps with the peak 521 forlight in the first polarization of the grating reflector 111. Hence, forlight at this frequency, the grating 111 selects the light in the secondpolarization to reflect while the grating 121 selects the light at thefirst polarization to reflect. The transmission peak 500 of the filter160 is set to overlap with both the peaks 512 and 521 to select a singlemode for laser oscillation at this laser frequency that has the properpolarization states at the grating reflectors 111 and 121.

Hence, in the above fiber laser 100, the laser cavity selects light atthe laser wavelength (λ_(L)) generated by the gain medium 150 to beamplified by their states of polarization. Detailed analysis shows thatthe circularly polarized light at the laser wavelength (λ_(L))experiences the minimum loss in the laser cavity shown in FIG. 1 (withthe associated components spectra detailed in FIG. 5) and hence onlycircularly polarized light at the laser wavelength (λ_(L)) will beselected by the cavity to amplify as the laser output. This selection bythe laser cavity in FIG. 1 may be understood by the fact that eachreflector 111 or 121 is selective in both wavelength and polarizationdue to the optical birefringence. Hence, the reflectivity of eachreflector 111 or 121 is highest and the associated optical loss isminimum when the light incident from the intracavity region is linearlypolarized along one principal polarization axis of the PM fiber andsatisfies the Bragg condition of the fiber grating 111 or 121. With theabove-described cavity configuration with the waveplates 140 and 170, acircularly polarized light between the waveplates 140 and 170, eitherright-handed or left-handed circularly polarized, meets suchrequirements. Hence, after the laser oscillation is established, thestates of polarization in the laser 100 are as follows: the laser outputis linearly polarized; the light between the waveplates 140 and 170 arecircularly polarized; the counter-propagating waves in the laser 100have mutually-orthogonal circular polarizations; and the intracavitylaser light is linearly polarized between the reflector 111 and thewaveplate 140, between the reflector 121 and the waveplate. Under thiscondition, there is no interference between counter-propagating waves.Therefore, distinctly different from other linear cavities, intracavitylaser light circulates in the linear cavity in FIG. 1 between thereflectors 111 and 121 without requiring an actual optical ring. As aresult, the laser 100 with a linear cavity operates like a ring laser.

The above relative alignment between the spectral reflection peaks ofthe PM fiber gratings 111 and 121 is to avoid the situation where thetwo reflection peaks from the grating reflector 111 simultaneously alignwith the two reflection peaks from the grating 121, respectively. Thiscondition may not be necessary if (1) the filter 160 selects only onepeak to transmit while rejecting the other peak; or (2) the PM fibergratings 111 and 121 have different birefringent properties. Under thesituation (2), when the PM fiber gratings 111 and 121 are birefringentlydifferent, the separation between the two reflection peaks for theorthogonal polarizations for the fiber grating reflector 111 can bedifferent from that in the fiber grating reflector 121. Under thiscondition, as long as the grating reflectors 111 and 121 have only onecommon reflection peak, the composite cavity effects due to the presenceof the filter 160 can be eliminated. Unlike the implementationillustrated in FIG. 5, the polarization for the grating reflector 111and the polarization for the grating reflector 121 in the overlappedreflection peaks may be the same, i.e., along either x or y directions,or different with one along the x direction and the other along the ydirection. However implemented, each of the polarization-sensitivereflectors 111 and 121 is designed to work with the corresponding waveplate (140 or 170) so that a reflected beam at the laser wavelength willnot be reflected by the reflector.

The above fiber laser has a number of advantages. For example, the lasermay be implemented in an all-fiber configuration where each component ismade of a fiber component. The laser can operate in a single frequencymode and the output laser frequency can be nearly independent on thepump power. As will be described below, the laser operating frequencycan be stabilized at a desired frequency by thermal compensation, e.g.,using a passive thermal compensation technique with negative thermalexpansion packaging materials. The output polarization of the abovelaser can be well defined and aligned with the principal axis of theoutput coupling fiber. In addition, a narrow optical passband filtersuch as a Fabry-Perot filter or a composite filter may be used tonarrowly define the laser frequency and stabilize the laser frequency ata desired laser frequency such as an ITU channel frequency. Furthermore,due to elimination of spatial hole burning, high side mode suppressionand signal to noise ratio may be achieved in such lasers.

In other implementations, the fiber laser 100 may be made tunable in thelaser wavelength as shown by an example in FIG. 2. The tunable fiberlaser 200 includes a fiber laser 100 and a cavity control unit-210. Theunit 210 interacts with the laser cavity of the laser 100 via a stimulus220 to adjust the laser frequency in the output 105. For example, theunit 210 may be a thermal control unit to control the temperature of thefiber cavity of the laser 100 where the optical path length of thecavity is tuned by changing the index of each fiber segment and thelength of each fiber. For another example, the unit 210 may be a fiberstretcher or compressor which applies a force to change the opticallength of the fiber cavity for tuning the laser wavelength.

FIG. 3 further shows that the fiber laser 100 may be packaged in anathermal design to stabilize the laser cavity against a change intemperature without active thermal control. In this design, the fiberlaser 100 may be mounted at two locations 310 and 320 to a temperaturecompensating mount 301 which has a negative thermal expansioncoefficient so that the effects of the temperature variations in thefiber laser 100 and in the mount 301 compensate each other to reduce theoverall effect on the laser cavity. The temperature compensating mount301 may be a ceramic or other suitable materials. The fiber laser 100may be mounted under a tension between locations 310 and 320. Fibers ofdifferent compositions and therefore different thermal properties areused to construct the fiber laser 100 and accordingly appropriatematerials with corresponding thermal expansion coefficients can be usedto compensate different sections of the laser 100.

FIG. 6A shows an exemplary cascaded fiber laser 600 where two or morefiber lasers 610 and 620 may be connected in series in a strand of fiberto share the same input optical pump beam 101. The two lasers 610 and620 may operate at different laser wavelengths λ1 and λ2 so that thelaser output 611 at λ1 from the laser 610 may transmit through the laser620 at λ2. The transmitted pump 101A of the laser 610 becomes the inputpump for the laser 620. The remaining transmitted pump beam 101B of thelaser 620 may be used to pump yet another laser. Different lasers 610and 620 may be respectively locked to different ITU WDM grids to produceWDM signals with a single pump beam 101.

As another variation, FIG. 6B shows an extra strand of doped fiberamplifier 630 at the output of the laser 610 to optically amplifier thelaser output 611 from the laser 610. An EDFA may be used as theamplifier 630 after the output grating 121 in the laser 100 in FIG. 1 inorder to boost the output power. This additional EDFA is pumped by thepump beam 101A that transmits through the laser 610 as long as the inputpump 101 is sufficiently strong. No additional optical pump is neededfor the extra EDFA in this design.

FIG. 7 shows an alternative implementation 700 of a traveling-wave laserwith a linear cavity for producing tunable laser output. In thisimplementation, the waveplates 140 and 170 are shown to be formed withinthe PM fibers 110 and 120 as described above in one implementation.Different from the laser 100 in FIG. 1, the reflectors 710 and 720 inlaser 700 are a sampled fiber grating and a tunable fiber grating,respectively. The sampled fiber-grating reflector 710 produces a seriesevenly-spaced reflection bands respectively centered at the ITU WDMgrid. The tunable fiber grating 720 is tunable to change its reflectionband from one WDM channel to another so that the laser 700 is tuned tolase at different WDM wavelengths. Also different from the laser 100 inFIG. 1, instead of using a single Fabry-Perot filter formed from twofiber gratings, a fiber comb filter 730 is used to produce a series ofnarrow and separated transmission peaks on the WDM grids to select oneWDM wavelength to lase at a time. The laser 700 is similar to the laser100 in FIG. 1 in that the two or more lasers operating at differentlaser wavelengths based on the design in FIG. 7 may be cascaded to sharethe same pump laser as shown in FIG. 6A and in that a down stream fiberamplifier may be added at the laser output as shown in FIG. 6B toamplifier the laser output based on the optical pumping by the pumplight transmitted through the laser.

In one implementation, the sampled fiber grating 710 is formed by havingtwo spatially overlapping spatial patterns with different spatialperiods in the PM fiber 110. The first spatial pattern in the fiber 110in the sampled fiber grating 710 has a grating period less than that ofthe second spatial pattern and thus operates as the underlying Braggreflection grating to produce a single Bragg reflection band. The secondspatial pattern overlaps with the first spatial pattern such that finalspatial pattern produced by the two overlapping spatial patterns is amultiplication of a first spatial modulation and a second specialmodulation. The coupling between the two spatial patterns produces aplurality of Bragg reflection bands at different wavelengths and with abandwidth determined by the first spatial pattern, i.e., the signalBragg reflection band is duplicated in the spectral domain at evenlyspaced locations. For WDM applications, the second spatial pattern maybe designed to place the Brag reflection bands at ITU WDM grids. FIG. 8shows the Bragg reflection spectrum of one exemplary sampled gratingreflector.

The comb filter 730 may be formed by various techniques. In oneimplementation, the comb filter 730 may be formed by two fiber gratingsin the fiber where their spatial grating patterns are spatially shiftedfrom each other. The amount of the spatial shift between the two gratingpatterns is selected to achieve a desired spectral separation ofadjacent transmission peaks of the comb filter 730. For WDMapplications, the transmission peaks respectively overlap with the ITUWDM grids or a fraction thereof.

One simple implementation of the comb filter 730 is to have a broadbandfiber Bragg grating separated by a small gap, which determines the freespectral range; such single pitch grating may not have wide enoughbandwidth. Another implementation of such a comb filter with much widerspectral width is to have two chirped gratings separated by a gap, whichwill define the free spectral range of such distributed FP cavity. Thetwo gratings may also partially overlap each other to extend thebandwidth of the comb. FIG. 9 shows the transmission spectrum generatedby such a fiber grating comb filter formed of two chirped grating with achirp rate of 1 nm/mm. The two gratings are written on top of each otherwith a 1 mm gap. This generates a comb filter with 100 GHz channelspacing anchored close to the ITU channels frequency.

In operation, the tunable fiber grating reflector 720 may be tuned tohave its Bragg reflection band aligned with a desired transmission peakof the comb filter 730 and one of the Bragg reflection band of thesampled grating reflector 710. Under this condition, the laser 700selects the wavelength where the elements 710, 720, and 730 spectrallyoverlap to lase. FIG. 10 illustrates one example of such spectraloverlap of the elements 710, 720, and 730 and shows the Comb filtertransmission spectrum in red created by the distributed FP filter, thesampled reflector spectrum due to splitting in PM fiber in solid blueand green lines, and the reflection spectrum of the output tuninggrating in dashed blue and green lines.

The tunability of the fiber-grating reflector 720 may be achieved invarious ways. In one implementation, the temperature of thefiber-grating reflector 720 may be controlled to tune the spectralposition of the Bragg reflection band and thus the laser wavelength ofthe laser 700. In another implementation, a fiber stretcher may beengaged to the fiber-grating reflector 720 to change the length of thefiber-grating reflector 730 by stretching or compressing for tuning thelaser wavelength. In yet another implementation, the underlying fibermay be designed to exhibit electro-optic effect so that an externalcontrol electric field may be used to tune the fiber grating.

Notably, the tunable fiber-grating reflector 720 may include a singlefiber grating or two or more fiber gratings that produce different Braggreflection bands. The spectral tuning range of a single-gratingreflector may be limited because the amount of change in the fiberlength or the index of the fiber is limited. In the latter configurationwith two or more fiber gratings, a wider tunable range may be achievedin comparison with a single-grating reflector because the Braggreflection bands from different underlying gratings may be used.

The above linear cavity designs and the associated lasers may also beimplemented with wave-guides formed on substrates such as planarwaveguides, where each fiber Bragg grating is accordingly replaced withan equivalent grating structure formed in a respective waveguidesection. Such wave-guides can be made from various waveguide materials.

In general, the wave plates 140 and 170 are polarization rotatingelements to rotate the polarization so that the counter-propagatinglaser beams between the wave plates 140 and 170 are orthogonal to eachother in polarization. In the above examples, the quarter wave plate 140and the three quarter wave plate 170 are used. FIG. 11A illustrates twoexemplary relative orientations of the wave plates 140 and 170 based onthe polarization directions of the reflectors 111 and 121. In each case,the wave plate 140 is oriented relative to the polarization direction ofthe polarization sensitive reflector 111 while the wave plate 170 isoriented relative to the polarization direction of the polarizationsensitive reflector 121. Assuming the reflector 111 is polarized alongthe x1 direction and the reflector 121 is polarized along the x2direction, the laser light reflected from the reflector 111 is linearlypolarized along the x1 direction when incident to the quarter wave plate140 and the laser light reflected from the reflector 121 is linearlypolarized along the x1 direction when incident to the three quarter waveplate 170. The relative orientations of the polarization-sensitivereflectors 111 and 121 are not restricted here. Hence, in general, x1may be at any angle with respect to x2. Accordingly, the relativeorientations of the wave plates 140 and 170 are not restricted either.Under this configuration, the quarter wave plate 140 has one principalaxis (e.g., the fast axis as in FIG. 11A) oriented at 45 degrees withrespect to the x1 direction while the three quarter wave plate 170 hasone principal axis (e.g., the fast axis as in FIG. 11A) oriented at 45degrees with respect to the x2 direction. FIG. 11B shows an alternativerelative orientation when both wave plates 140 and 170 are quarter waveplates.

In another implementation, the three quarter wave plate 170 may bereplaced by a second quarter wave plate to make the polarizations ofcounter-propagating laser beams between them orthogonal. In yet anotherimplementation, both waveplates may be three quarter waveplates to makethe polarizations of counter-propagating laser beams between themorthogonal.

FIGS. 12A and 12B show two relative orientations for this design. InFIG. 12A, the fast axis of the quarter wave plate 140 is at 45 degreeswith respect to the x1 direction of the grating reflector 111 while thefast axis of the quarter wave plate 170 is at −45 degrees with respectto the x2 direction of the grating reflector 121. FIG. 12B shows anotherorientation configuration under this design. If x1 and x2 are parallel,the slow axis of the first quarter wave plate 120 is perpendicular tothe slow axis of the second quarter wave plate that replaces the threequarter wave plate 170 in FIG. 1 and one of its principal axes, e.g.,the slow axis, of the first quarter wave plate 120 forms 45 degrees withrespect to the x direction.

As described above, the lasers described above may be implementedwithout the intracavity optical filter 160. FIGS. 13A and 13B show twoexemplary lasers 1300A using one quarter wave plate 140 and a threequarter waveplate 170 and 1300B using two quarter waveplates 140 and1320. Certainly, the positions of the using one quarter wave plate 140and a three quarter waveplate 170 in FIG. 13A can be exchanged.

In the above described lasers, PM fiber sections 110 and 120 are used onboth sides of each laser so that the fiber grating reflectors 111 and121 are polarization selective in reflecting light. As an alternativeimplementation, the input section may be implemented by a non-PM fibersection. FIG. 14 illustrates one example 1400 where a non-PM fibersection 1410 is joined with the quarter waveplate 140 at the connectingpoint 112. As in previous examples, one of the two waveplates 140 and1320 in FIG. 14 may be replaced by a three quarter waveplate. Inaddition, both waveplates 140 and 1320 may be three quarter waveplates.In the non-PM fiber section 1410, a non-PM fiber grating reflector 1411is formed to reflect light at the laser wavelength and to transmit lightat the pump wavelength. Notably, because the fiber grating reflector1411 is no longer a PM device, light at the laser wavelength indifferent polarizations can be reflected back into the laser cavityformed between the reflectors 1411 and 121. The fiber section 120 at theoutput of the laser 1400 and the fiber grating reflector 121 remain asPM devices and thus select only one polarization to reflect back to thelaser cavity. The non-PM fiber grating reflector 1411 may configured tobe totally or nearly totally reflective to light at the laser wavelengthwhile the PM fiber grating reflector 121 is a partial reflector to lightat the laser wavelength to produce a laser output 105. An optionalintracavity filter 160 may be implemented inside the cavity in FIG. 14.

The above examples of lasers use a single-mode fiber section 130 as thelaser gain section between the two waveplates 140 and 170 in FIGS. 1, 7and 13A and 140 and 1320 in FIGS. 13B and 14. This single-mode fibersection 130 is ideally free of optical birefringence or itsbirefringence is sufficiently small so that the light propagation in thesection 130 does not alter the polarization state of light. In practicalimplementations, various optical gain media exhibit some degree ofoptical birefringence and when the optical birefringence in the section130 is sufficient to alter the polarization state of light, the opticalloss in the lasers described above can increase. Hence, the opticalbirefringence in the gain section 130 between the waveplates isundesirable for the laser configurations described above.

FIGS. 15 and 16 illustrate two laser designs that allow for opticalbirefringence in the laser gain section.

FIG. 15 shows a laser 1500 that includes non-PM fiber sections 1510 and1520 for the input and output and a fiber section 1530 with abirefringent gain medium 1532 between the non-PM fiber sections 1510 and1520. Accordingly, the two fiber grating reflectors 1511 and 1521 arenon-PM fiber grating reflectors. In this particular example, two quarterwaveplates 140 and 1320 are connected at the intersection between theinput non-PM fiber section 1510 and the fiber section 1530 and theinteraction between the output non-PM fiber section 1520 and the fibersection 1530, respectively. The two quarter waveplates 140 and 1320 maybe replaced by either (1) two three quarter waveplates or (2) onequarter waveplate and one three quarter waveplate. The fiber section1530 includes three parts: a first PM fiber section 1531 connected tothe waveplate 140, the birefringent gain medium 1532 such as a PM fibergrain section and a second PM fiber section 1533 connected to the otherwaveplate 1320. The relative orientation of the principal axis of eachwaveplate and the respective connected PM fiber section is shown in FIG.12A or 12B and thus operates to ensure the counter-propagating lightwaves at the laser wavelength within the fiber section 1530 areorthogonal to each other in polarization. Different from the previouslaser designs, the counter-propagating light waves at the laserwavelength within the fiber section 1530 are linearly polarized ratherthan circularly polarized because the two fiber grating reflectors 1511and 1521 are not polarization selective and reflector light in allpolarizations.

The laser 1500 in FIG. 15 is different other lasers described inprevious sections in that an intracavity filter 160, if inserted in thefiber section 1530 between the two waveplates 140 and 1320, can lead toundesired multiple composite cavities due to reflections by the fiber160. Hence, the laser 1500 in FIG. 15 can be implemented without theintracavity optical filter 160. As an option, the output non-PM fibersection 1520 may be coupled to an output PM-fiber section 1540 whichincludes a quarter waveplate or a three quarter waveplate 1550 orientedto convert the linearly polarized laser output into circularly polarizedlaser output. Certainly, multiple lasers based on the design in FIG. 15may be cascaded in series to share a single optical pump in a similarconfiguration as described in previous sections.

Referring now to FIG. 16, the laser 1600 includes two PM fiber sections110 and 120 as in FIG. 1 but a fiber section 1630 with a birefringentgain medium 1630 between the PM fiber sections 110 and 120. Similar toFIG. 15, two PM fiber sections 1631 and 1633 are connected on two sidesof the birefringent gain medium 1632 and are connected to two differentwaveplates 140 and 1620. Also similar to FIG. 15, the waveplate 140 maybe a quarter waveplate or a three quarter waveplate with a relativeorientation shown in FIG. 12A or 12B. However, the waveplate 1620 isspecially designed to have an optical birefringence equal to a threequarter waveplate or a quarter waveplate less than the opticalbirefringence of the birefringent gain medium 1632. Assuming thebirefringence of the birefringent gain medium 1632 is Kλ or (N+K)λ whereK is a fraction and N is an integer, the birefringence of the waveplate1620 is (¾-K)λ or (¼-K)λ. In essence, the optical birefringence of thewaveplate 1620 is designed based on the birefringence n the gain section1632 to “offset” the optical birefringence in the gain section 1632 sothat the net result is similar to what is in laser FIG. 1 where twocounter-propagating laser waves are substantially or closely orthogonalto each other. Further different from the laser in FIG. 15, anintracavity optical filter 160 may be inserted between the waveplates140 and 1620 without creating undesired composite cavities with thefiber grating reflectors 111 and 121. Two or more lasers in FIG. 16 maybe cascaded in series to share a single pump laser. The positions of thetwo waveplates 140 and 1620 can be exchanged.

In the above fiber implementations, the fiber gain section 130, 1530 or1630 between the two waveplates may be a straight section fiber and maybe held under a tension as illustrated in the example shown in FIG. 3.To achieve a higher gain from such a fiber laser, the length of thefiber gain section 130, 1530 or 1630 can be increased. This extendedgain section 130, 1530 or 1630 may be undesirable when dimension of thefiber laser is to be limited to a compact size. One approach to a highergain fiber laser is to use a looped fiber gain section by winding alength of fiber doped with active laser ions into a small fiber loop.The bending of the fiber, however, can induce optical birefringence inthe fiber and thus alter the optical birefringence of the fiber gainsection. This change is undesirable for the laser designs describedabove and may be compensated using the technique in FIG. 15. One specialfiber known as a helical fiber or twisted single-mode fiber can be usedto avoid this problem. A twisted single-mode fiber is made of an opticalmaterial with elastooptically induced optical activity and under aproper amount of twist in the fiber when forming a fiber loop, theresultant fiber can preserve the polarization of light and thus behaveslike a birefringence-free optical medium. See, examples described in“Polarization optics of twisted single-mode fiber” by R. Ulrich and A.Simon in Applied Optics, Volume 18, No. 13, pages 2241-2251 (1979),which is incorporated herein by reference.

FIG. 17 illustrates a laser 1700 that uses a fiber loop 1710 as the gainmedium made from a loop of helical fiber or twisted single-mode fiberdoped with active laser ions. The fiber loop 1710 is configured topreserve the optical polarization of light. Hence, an extended lengthyof the doped fiber may be implemented with the fiber loop 1710 toachieve a high laser gain while still maintaining a compact size for thelaser 1700. Various implementations described above for other laserdesigns may be applied to laser 1700.

Only a few implementations and examples are disclosed. However, it isunderstood that variations and enhancements may be made. For example,the above laser designs may be implemented with non-fiber components.The laser gain section may be implemented with a semiconductor opticalamplifier, a solid-state laser material such as a laser crystal, a microdisk solid state optical amplifier such as a spinning crystal disk madeof YAG and other crystals, and photonic crystal fibers with largersingle-mode cross-section diameter to achieve higher laser power andimprove spatial overlap between the modes of the pump light and laserlight. Other variations are also possible.

1. A device, comprising: a linear optical cavity having first and secondoptical reflectors to reflect light at a laser wavelength and totransmit light at a pump wavelength different from the laser wavelength,wherein said first reflector selectively reflects only light linearlypolarized in a first direction and said second reflector selectivelyreflects light linearly polarized in a second direction; a laser gainsection in said linear optical cavity to produce optical gain at thelaser wavelength by absorbing the pump light; and first and secondoptical polarization elements in said linear optical cavity andrespectively located on two opposite sides of said laser gain section tomake counter-propagating light beams at the laser wavelength reflectedfrom said first and second optical reflectors to have orthogonalpolarizations between said first and second optical polarizationelements.
 2. The device as in claim 1, wherein said linear opticalcavity is a linear fiber cavity, and wherein said laser gain section isa doped fiber segment, and wherein said first and said second reflectorsare fiber Bragg gratings.
 3. The device as in claim 2, wherein saidfiber Bragg gratings are reflective to light at the laser wavelength andare transmissive to light at the pump wavelength.
 4. The device as inclaim 1, further comprising a pump reflecting fiber Bragg gratingpositioned to reflect light at the pump wavelength transmitting throughsaid laser gain section back to said laser gain section.
 5. The deviceas in claim 1, wherein said first polarization element is a quarter-waveplate and said second polarization element is a three-quarter-waveplate.
 6. The device as in claim 5, wherein said quarter-wave plate isoriented to have a principal axis at a 45 degree with respect to saidfirst direction, and wherein said three quarter wave plate is orientedto have a corresponding principal axis at 45 degrees with respect tosaid second direction.
 7. The device as in claim 1, wherein each of saidfirst and said second polarization elements is a quarter-wave plate. 8.The device as in claim 7, wherein said first polarization element isoriented to have a principal axis at a 45 degree with respect to onesaid first direction, and wherein a corresponding principal axis of saidsecond polarization element is at −45 degrees with respect said seconddirection.
 9. The device as in claim 1, further comprising a lasercontrol mechanism engaged to said linear optical cavity to adjust anoptical path length of said linear optical cavity in response to acontrol signal and to tune a wavelength of laser output from said linearoptical cavity.
 10. The device as in claim 1, wherein the firstdirection of said first optical reflector is orthogonal to the seconddirection of said second optical reflector.
 11. The device as in claim1, wherein the first direction of said first optical reflector isparallel to the second direction of said second optical reflector. 12.The device as in claim 1, wherein said laser gain section comprises asolid state laser disk as a gain medium.
 13. The device as in claim 1,wherein said laser gain section comprises a photonic crystal fiber as again medium.
 14. The device as in claim 1, wherein said laser gainsection comprises a semiconductor optical amplifier as a laser gainmedium.
 15. The device as in claim 1, wherein said doped fiber gainsection comprises at least one of a doped silica fiber, a dopedphosphate fiber, a doped fluoride fiber, and a doped bismuth fiber. 16.A device, comprising: a first polarization-maintaining (PM) fibersection having a first fiber grating to reflect light at a laserwavelength and to transmit light at a pump wavelength different from thelaser wavelength; a doped fiber gain section to produce optical gain atthe laser wavelength by absorbing light at the pump wavelength from saidfirst PM fiber section; a first wave plate optically coupled betweensaid first PM fiber section and a first side of said doped fiber gainsection and oriented to covert light from said first fiber grating intoa first circularly polarized light; a second PM fiber section having asecond fiber grating to reflect light at the laser wavelength and totransmit light at the pump wavelength; and a second wave plate opticallycoupled between said second PM fiber section and a second side of saiddoped fiber gain section, and oriented to convert light from said secondfiber grating into a second circularly polarized light orthogonal tosaid first circularly polarized light.
 17. The device as in claim 16,wherein said first PM fiber section is polarized along a firstpolarization direction and said second PM fiber section is polarizedalong a second polarization direction orthogonal to said firstpolarization direction.
 18. The device as in claim 16, wherein saidfirst PM fiber section is polarized along a first polarization directionand said second PM fiber section is polarized along a secondpolarization direction parallel to said first polarization direction.19. The device as in claim 16, wherein said doped fiber gain sectionexhibits an optical birefringence, and wherein one of said first andsaid second wave plates is configured to have an optical birefringencethat has a fixed relationship with said optical birefringence of saiddoped fiber gain section.
 20. The device as in claim 19, wherein saidone wave plate is configured to have an optical birefringence of aquarter wave plate or a three quarter waveplate less than said opticalbirefringence of said doped fiber gain section.
 21. A device,comprising: a first fiber section having a first fiber grating toreflect light at a laser wavelength and to transmit light at a pumpwavelength different from the laser wavelength, said first fiber sectionconfigured to reflect light of different polarizations; a doped fibergain section to produce optical gain at the laser wavelength byabsorbing light at the pump wavelength from said first fiber section; afirst wave plate optically coupled between said first fiber section anda first side of said doped fiber gain section and oriented to covertlight from said first fiber grating into a first polarized light; asecond fiber section made of a polarization-maintaining (PM) fiber andhaving a second fiber grating to reflect light at the laser wavelengthalong a polarization defined by said PM fiber and to transmit light atthe pump wavelength; and a second wave plate optically coupled betweensaid second PM fiber section and a second side of said doped fiber gainsection, and oriented to convert light from said second fiber gratinginto a second polarized light orthogonal to said first polarized light.22. A device, comprising: a first fiber section having a first fibergrating to reflect light at a laser wavelength and to transmit light ata pump wavelength different from the laser wavelength, said first fibersection configured to reflect light of different polarizations; a dopedfiber gain section exhibit optical birefringence and operable to produceoptical gain at the laser wavelength by absorbing light at the pumpwavelength from said first fiber section; a first wave plate opticallycoupled between said first fiber section and a first side of said dopedfiber gain section and oriented to covert light from said first fibergrating into a first polarized light; a second fiber section having asecond fiber grating to reflect light at the laser wavelength alongdifferent polarizations and to transmit light at the pump wavelength;and a second wave plate optically coupled between said second fibersection and a second side of said doped fiber gain section, and orientedto convert light from said second fiber grating into a second polarizedlight orthogonal to said first polarized light.
 23. The device as inclaim 1, comprising: an optical fiber segment in which the linearoptical cavity and the laser gain section are formed; and a mount onwhich the optical fiber segment is attached at two different locations,the mount structured to have an athermal structure that reduces a changein a spacing between the two locations caused by a change intemperature.
 24. The device as in claim 1, wherein the optical gainsection includes a loop of helical fiber or twisted single-mode fiberdoped with active laser ions.
 25. The device as in claim 16, wherein thefirst PM fiber section, the first waveplate, the doped fiber gainsection, the second waveplate and the second PM fiber section areconnected in series to form connected structure as an optical linearcavity, and wherein the device comprises: a mount on which the opticallinear cavity is attached at two different locations, the mountstructured to have a passive athermal structure that reduces a change ina spacing between the two locations caused by a change in temperature.26. The device as in claim 16, wherein the doped fiber gain sectionincludes a loop of helical fiber or twisted single-mode fiber doped withactive laser ions.
 27. The device as in claim 21, wherein the firstfiber section, the first waveplate, the doped fiber gain section, thesecond waveplate and the second fiber section are connected in series toform a connected structure as an optical linear cavity, and wherein thedevice comprises: a mount on which the optical linear cavity is attachedat two different locations, the mount structured to have a passiveathermal structure that reduces a change in a spacing between the twolocations caused by a change in temperature.
 28. The device as in claim21, wherein the doped fiber gain section includes a loop of helicalfiber or twisted single-mode fiber doped with active laser ions.
 29. Thedevice as in claim 22, wherein the first fiber section, the firstwaveplate, the doped fiber gain section, the second wave plate and thesecond fiber section are connected in series to form a connectedstructure as an optical linear cavity, and wherein the device comprises:a mount on which the optical linear cavity is attached at two differentlocations, the mount structured to have a passive athermal structurethat reduces a change in a spacing between the two locations caused by achange in temperature.